1. Field of the Invention
The invention is directed to an inductive flow meter.
2. Description of Related Art
Inductive flow meters have a magnetic field generator, e.g. a coil, for generating a laterally directed magnetic field in a tube. Further, the tube is provided with two electrodes the voltage of which is supplied to an evaluating circuit. When an electrically conductive liquid is flowing through the tube transversely to the magnetic field, a voltage is generated at the electrodes, the level of this voltage being proportional to the magnetic field strength and to the flow speed.
The first inductive flow meters have been devices operated by alternating fields, wherein the magnetic field generator is connected directly to the voltage of the power supply, for example 220 V/50 Hz.
An advantage of such an alternating-field device resides in that, without requiring too complex switching and control means, a strong magnetic field can be generated so that a large signal amplitide is obtained at the electrodes and thus a large signal-to-noise ratio can be reached. By the comparatively high frequency of the power supply voltage, an inductive flow meter of the described type has a fast response characteristic. However, the systems of alternating-field devices have the inherent disadvantage that the useful signal is subjected to a zero drift. The useful signal generated at the electrodes has a phase-shift of about 90.degree. with respect to the alternating voltage of the power supply. Since the evaluating circuit, along with the lines coming from the electrodes, is very susceptible to interferences, noise voltages originating directly in the power supply voltage as well as self-induced noise voltages are carried into the evaluating circuit. According to usual practice, the noise signals contained in the useful signal are eliminated by phase selection. However, since the phase relation between the noise signal and the useful signal varies with time in dependence of the temperature of the device and other influences, a phase-selective elimination of the noise signals necessitates frequent adjustment and re-adjustment. A further disadvantage consists in a considerable reactive-power consumption of alternating-field devices.
More recent inductive flow meters are operated with a clocked DC-current field. In these flow meters, the magnetic field generator is excited by a clocked DC-current, while the polarity of the DC-current excitation may alternate. Devices of this type have excellent zero stability and low power consumption. Nevertheless, it is a massive disadvantage of these devices that, using electronic means merely to a reasonable extent, only relatively small exciting currents can be generated so that the amplitude of the obtained useful signals is also small. The clock frequency of the clocked DC-current field is comparatively low so that the reaction speed of the flow metering is reduced. Both, the relatively low useful voltage and the low clock frequency, cause a small signal-to-noise ratio.
Further, an inductive flow meter is known from German Patent Application 20 54 624. In this flow meter, the exciter coil generating the magnetic field is contained in a bridge circuit consisting of four electronic switches and being connected to the supply voltage. At each time, two diagonally arranged electronic switches of the bridge circuit are in the conducting state. By switching the pairs of switches, the polarity of the exciter coil can be reversed with respect to the supply voltage. This reversal of polarity is performed respectively at a zero crossing of the alternating current. Since the electronic switches are triacs, they are always extinguished at the zero crossing of the exciting current and then are immediately ignited again. Each of the two pairs of switches remains active over a specific number of periods which is detected by a counter. Then, the other pair of switches is excited. The periodical reversal of polarity of the exciter coil with respect to the supply voltage is provided to compensate those noise voltages which, with respect to their amplitude and their phase length, undergo a slow change over time. The evaluating circuit is provided with a rectifier which receives a phase reference signal and admits only that component of the electrode signal which is in phase with a phase reference signal. Also this arrangement involves the above-mentioned disadvantages of alternating-field devices, i.e. this method has no inherent zero accuracy (autozero).
German Patent Application 27 25 026 discloses also a alternating-field device, wherein the magnetic field exciter is connected to the alternating current voltage via an electronic switch. The electronic switch is clocked by a clock-pulse generator having a fixed clock ratio, the frequency of the clock-pulse generator being a multiple of the supply current frequency. The clockpulse generator is operated independently of the signals of the evaluating circuit so that the timing of the switch actuation is in no predetermined phase relation to the frequency of the alternating current. The electrode signals, having been amplified, are supplied to sample and hold circuits the output signals of which are subtracted from each other. Also the system of this method does not provide zero accuracy because the noise component, generated by self-induction, is not eliminated by the subtraction.
It is the object of the invention to provide an inductive flow meter which combines the advantages of the device operated by an alternating-field and the devices using a clocked DC-current field without taking on the disadvantages of those two types of devices. This means that the inductive flow meter is to be operated using AC-voltage and without excessive control efforts while, at the same time, avoiding the danger of zero drift, i.e. in this device zero accuracy (autozero) shall be obtained as a feature inherent to the system (and not by compensation or adjustment).